1. Field of the Invention
This invention relates to an image recording medium which stores electric charges as latent image charges which are generated by recording electromagnetic waves carrying image information and a method of producing the same.
2. Description of the Related Art
As an image recording medium which stores electric charges, as latent image charges, generated upon exposure to recording electromagnetic waves carrying image information, there has been known a radiation image recording medium comprising a photoconductive body such as a selenium plate sensitive to a radiation such as X-rays. The radiation image recording medium is used, for instance, in taking a diagnostic radiation image and is exposed to X-rays passing through an object. Upon exposure to X-rays carrying thereon radiation image information, each part of the radiation image recording medium stores an electric charge according to the amount of X-rays to which the part is exposed, whereby the radiation image recording medium stores, as an electrostatic latent radiation image, the radiation image information carried by the X-rays. The radiation image information can be read out from the radiation image recording medium by causing a laser beam or a line beam to scan the radiation image recording medium. See, for instance, U.S. Pat. No. 4,535,468. By the use of the radiation image recording medium, the irradiation dose to the patient can be reduced and at the same time, the diagnostic performance can be improved.
We, this applicant, have proposed, in U.S. Pat. No. 6,268,614 and Japanese Unexamined Patent Publication No. 2000-16726, a radiation image recording medium which enables high response in reading and efficient takeout of the signal charge to be compatible with each other, a method of and an apparatus for recording a radiation image on the radiation image recording medium, and a method of and an apparatus for reading out a radiation image from the radiation image recording medium.
In the method of recording and reading out a radiation image disclosed in U.S. Pat. No. 6,268,614, a radiation image recording medium comprising a first electrode layer permeable to recording radiation or light generated by excitation by the recording radiation, a recording photoconductive layer which exhibits conductivity upon exposure to the recording radiation or the light generated by excitation by the recording radiation, a charge transfer layer which behaves like a substantially insulating material to a latent image charge and behaves like a substantially conductive material to a transfer charge in the polarity opposite to the latent image charge, a reading photoconductive layer which exhibits conductivity upon exposure to reading electromagnetic wave and a second electrode layer permeable to the reading light which are superposed one on another in this order is used, and the recording radiation is projected onto the first electrode layer of the radiation image recording medium so that electric charges are stored in a charge storing portion formed in an interface between the recording photoconductive layer and the charge transfer layer in an amount corresponding to the amount of the recording radiation projected onto the first electrode layer, whereby radiation image information is recorded on the radiation recording medium as an electrostatic latent image. Then the reading electromagnetic wave is projected onto the radiation image recording medium to read out the electrostatic latent image, whereby an image signal representing the radiation image information is obtained.
Further, we, this applicant, have proposed a radiation image recording medium in which the second electrode layer is in the form of a stripe electrode formed by a plurality of linear electrodes which are permeable to the reading electromagnetic wave and are arranged like stripes.
When radiation passing through an object is projected onto the first electrode layer of the radiation image recording medium with a DC voltage applied to the radiation image recording medium so that the first and second electrode layers respectively have negative and positive potentials, charged pairs are generated in the recording photoconductive layer in an amount corresponding to the amount of radiation passing through the first electrode layer, and negative charges are stored in the charge storing portion as latent image charges, whereby radiation image information is recorded on the radiation image recording medium as an electrostatic latent image.
When the reading electromagnetic wave is projected onto the second electrode layer of the radiation image recording medium after application of the DC voltage is stopped and the first and second electrode layers are short-circuited to relocate the charges, the reading electromagnetic wave passes through the second electrode layer and impinges upon the reading photoconductive layer to generate charged pairs in the reading photoconductive layer. The positive charges of the charged pairs passes through the charge transfer layer and combine with the negative charges stored in the charge storing portion whereas the negative charges recombine with the positive charge in the second electrode layer, whereby electrical discharges occur. By detecting change of voltage between the first and second electrode layers generated by the electrical discharges as electric current change by a current amplifier or the like, an image signal representing the radiation image information can be obtained.
An image is reproduced on the basis of the image signal on a display such as a monitor and used as a diagnostic image.
However, the radiation image recording medium is disadvantageous in that when a high voltage is applied between the first and second electrode layers, there is a fear that creeping discharges occur on the periphery of the first electrode layer to break the electrode, which can result in a defective reproduction image. There has been proposed in U.S. Pat. No. 6,495,817 to prevent breakage of the electrode due to creeping discharge by covering the entire area of a thick amorphous semiconductor film (corresponding to the recording photoconductive layer) with a highly resistive film. However, when a highly resistive film is provided in such a manner, an image area (an area actually exposed to the recording light) other than the periphery of the first electrode layer where creeping discharge can actually occur is also covered with the highly resistive film, which can cause chemical change in quality of the first electrode layer and/or the recording photoconductive layer and absorption of the recording light by the highly resistive film, thereby deteriorating the quality of the reproduction image.